The invention relates to a digital subscriber line circuit for connecting an ISDN subscriber to the truck side of a digital exchange, comprising an SLD interface operating according to the SLD system and a transfer arrangement for exchanging, during a transfer time window, data signals between incoming and outgoing time slots of a SL line of the SLD interface and time slots assigned thereto of outgoing or incoming trunks (highways) of the digital exchange.
The generally known standard SLD interface has thusfar been used on the extension side of a digital exchange and operates in the so-called "master" mode. In this mode the internal SCL clock signal of the SLD interface controls the timing when SLD data signals are transmitted.
When such an SLD interface is used on the trunk side of a digital exchange, problems arise because the interanl SCL clock signal of the SLD interface, which signal controls the data transmission on the SLD interface, does not synchronize with the digital exchange clock signal which controls the trunk interface on the side of the digital exchange. The problem manifests itself as so-called slip, that is to say, that the time slots of the outgoing or incoming trunks of the digital exchange extend to beyond the transfer time window, so that a correct transfer of data signals between incoming and outgoing time slots of the SL line of the SLD interface and assigned time slots of outgoing or incoming trunks respectively, of the digital exchange is no longer possible. Data losses frequently occur during the slip period.